1. Field of the Invention
This invention relates to an automatic exposure control camera having a low brightness warning device.
2. Description of the Prior Art
The metering systems for automatic exposure control of single lens reflex cameras include, for example, the TTL open metering system disclosed in U.S. Pat. No. 3,641,890 and the TTL stop-down metering system disclosed in U.S. Pat. No. 4,204,755. In the automatic exposure control by the TTL open metering system, open metering is effected before exposure control and the metering value is stored, and then the aperture value, the shutter time or the like is controlled by the stored value. On the other hand, in the automatic exposure control by the TTL stop-down metering system, where the aperture is to be controlled, the brightness varying as the aperture is stopped down from the open diameter to the small aperture diameter is measured in real time and when it has become a predetermined amount, the stop-down is restrained and the metering value after the restraining of the stop is stored, and control of the shutter time is effected on the basis of the stored value. As compared with the former, the latter is preferable because, during automatic exposure, the storage is effected after the aperture has been determined and any error in the aperture diameter of the lens is automatically corrected by the shutter time and accordingly, a proper exposure is always obtained.
In the dynamic range of automatic exposure control relative to a variation in the brightness of the light from an object to be photographed, there are limits resulting from two factors. A first factor is the presence of a limit in the aperture value control range or the shutter time control range of the camera, namely, a limit in exposure control. A second factor is the presence of a limit in the dynamic range of a light-receiving element such as a silicon photodiode or cadmium sulfide used in the metering circuit and in the dynamic range of a logarithmic converting circuit which deals with the output of the light-receiving element. This limit by the dynamic range, particularly, the limit on the low brightness side of the brightness of the light from the object passed through the stop, is attributable to the aggravation of the SN ratio which in turn, chiefly in the light-receiving element, is attributable to the dark current and the dark resistance, and to the aggravation of the SN ratio which in turn, on the circuit side, is attributable to the finite input impedance and the stray current such as leak current or the like.
Also, on the high brightness side, a limit of the dynamic range arises from the deviation from the expected rectilinearity which is attributable to the fact that the serial resistance components in the light-receiving element and the output impedances of the circuits used on the circuit side are finite.
Accordingly, the range which can be followed up in automatic exposure control must not exceed the limits resulting from the first and second factors.
However, in a camera of the type in which exposure display is accomplished by open metering and exposure control is accomplished by stop-down metering, there arises the disadvantage that even if, during exposure display, the limit resulting from the second factor is not exceeded and no warning is effected, when exposure control is carried out in such state, stop-down takes place to decrease the light incident on the light-receiving element and the limit resulting from the second factor is exceeded to permit inaccurate exposure control to take place.